A thermal asperity (TA) is an unwanted event sometimes occurring within hard disk drives employing a magneto-resistive (MR) read head element of a flying head or slider assembly. One way to increase data storage densities in magnetic hard disk drives having disks of a given size is to "fly" the head structure closer and closer to the disk surface than heretofore. Flying heights of one micro-inch are now being approached in hard disk drives. Unfortunately, disk surfaces are not perfectly smooth when viewed in a micro-inch domain. Also, occasional particulate contaminants may come between the slider and the disk surface.
When an MR read element comes into contact with a rough spot of the disk, or collides with a minute, freely moving contaminating particle, virtually instantaneous heating of the MR element occurs. In response to this sudden heating, resistance of the thin film MR stripe element quickly increases. Since a constant bias voltage or current is applied to bias the MR element during reading operations, the sudden increase in resistance is sensed by a preamplifier as a dramatic, rapid change in bias voltage, or a large baseline signal shift. This unwanted electrical signal shift (referred to hereinafter as a "thermal asperity" or "TA") can be several times larger than a signal shift induced by magneto-resistance effect in response to a magnetic flux transition recorded on the disk as user or servo data, etc.
Since the head preamplifier has a dynamic operating range optimized to expected signal magnitudes attributable to flux transitions, the out-of-range TA signal shift causes the electronics of the read channel to saturate. Once the read channel saturates, a relatively long time is needed to bring the channel back to nominal baseline operating conditions, and the saturation may remain over several subsequent data bit cells. Under a saturation condition, the read channel electronics of the disk drive cannot detect any recorded data transitions accurately.
There have been several prior approaches directed to solving or ameliorating the TA signal shift problem. One prior approach has focused upon changing the general characteristics of the read channel dynamics. In particular, one approach has been to increase the low frequency pole of the read channel bandpass. This approach is suggested in U.S. Pat. No. 5,233,482 to Galbraith et al, entitled: "Thermal Asperity Compensation for PRML Data Detection". This prior approach also included altering the dynamic operating range of a signal sampling analog-to-digital converter. Although this prior approach is relatively simple to implement, it does not provide comprehensive protection against data loss attributable to TA baseline signal shift.
Another approach has been to "subtract" out the TA baseline signal shift. However, the subtraction approach requires significant additional, complex and expensive electronics.
Another approach, described in U.S. Pat. No. 5,497,111 to Cunningham, entitled: "Peak Detection Circuit for Suppressing Magnetoresistive Thermal Asperity Transients in a Data Channel", calls for differentiating the incoming signal prior to gain normalization, and then passing the resultant signal through a modified amplitude qualification circuit to extract output pulses. One drawback of that approach is that since the incoming signal is differentiated before being gain-normalized, noise events, as well as magnetic flux transitions, are passed by the circuit, to a conventional ECC process as a lone safeguard to protect data integrity.
Thus, a hitherto unsolved need has remained for a more effective method and apparatus for recovering data from a magnetic recording disk with an MR head in the presence of thermal asperities.